A major goal of neurobiological research is the understanding of the underlying organization of neurotransmitter and receptor systems related to the function of the nervous system. One approach to this problem is to study the synaptic physiology of a well-documented neural circuit that subserves a specific behavior. This approach has been successfully applied to an analysis of the relative simple nervous systems in many invertebrates. One preparation that has been useful in a cellular analysis of behavior is the marine mollusk Hermissenda. The neural circuit underlying a well-defined and quantified example of behavior will be studied using a combination of cellular neurophysiological and pharmacological techniques. Using intracellular recording and stimulation techniques putative motor neurons and interneurons that receive synaptic input from the visual system will be identified and the response to light classified. The relative contribution of identified pedal neurons and interneurons to the network underlying phototactic behavior will be studied in intact animals after eliminating specific neurons by the photoinactivation technique. Changes in behavior will be assessed following the selective elimination of specific neurons in the neuronal circuit including specific photoreceptor types, interneurons, and putative motor neurons. The neurotransmitters mediating the synaptic interactions among the neurons of this network will be examined using pharmacological and electrophysiological techniques. The role of biogenic amines such as dopamine and serotonin in modulating the activity of primary sensory neurons and putative motor neurons will be studied in the isolated nervous system. Since peptides may occur together with other neurotransmitters, the effects of various peptides on the photoresponse will be examined. An understanding of the neurotransmitter and receptor systems in a relatively simple neural network and its modulation by biogenic amines and peptides will provide a useful model system for studies of affective disorders in more complex mammalian nervous systems.